2005-01-12 04:00:00 PDT San Diego -- Astronomers reported Tuesday that they had convincingly detected, in the patterns of galaxies scattered across the night sky, the vestiges of sound waves that rumbled through the universe after the Big Bang.

Stars and galaxies tended to form along the ripples of the sound waves where matter was slightly denser, and the pull of gravity was slightly stronger. The ripples preserve a picture of the universe when it was only about 1 million years old and fit well with astronomers' ideas of how the universe, which started smooth and uniform, became lumpy with stars, gas clouds and other celestial objects.

Two teams of researchers analyzing the locations of thousands of galaxies from two sections of the sky reported similar findings on the sound waves at a meeting of the American Astronomical Society in San Diego. Earlier research had found signs of the ripples, but "we regard this as smoking-gun evidence," said Dr. Daniel Eisenstein of the University of Arizona, lead investigator of one of the teams.

"The important picture we have of the universe is hanging together amazingly well," said Dr. Martin Rees, a professor of cosmology and astrophysics at Cambridge University who was not involved with either team. "The standard picture is firming up."

According to that picture, matter was evenly distributed in all directions for the first instant after the Big Bang. But then burbling caused by the physics of quantum mechanics created slight imperfections, clumps that were slightly denser with ordinary matter, as well as dark matter, the unknown material that accounts for most of the mass in the universe.

Just as ripples spread out from a pebble dropped in a pond, sound waves spread out from the dense clumps through the hot gas made of matter, which is composed of electrons and protons, and of photons, or particles of light.

About 400,000 years after the Big Bang, the universe cooled enough that the charged electrons and protons combined to form hydrogen atoms, which allowed most of the photons to escape the hot gas. Several years ago, astronomers detected the sound waves etched by the photons.

The sound waves continued to spread for an additional 600,000 years, and when the last remaining photons escaped, the waves stopped, roughly 500,000 light-years from the dense clumps that produced them. When stars began to form, they tended to form around either the clumps of dark matter or along the ripples.

As the universe has expanded in the 13.7 billion years since then, the typical distance between ripple and clump has stretched to 500 million light- years.

The new research shows the matter component of the early sound waves. Galaxies in the present universe are more likely to be 500 million light-years apart than other distances, Eisenstein said. One light-year is the distance light travels in a year, or 6 trillion miles.

Eisenstein and his colleagues used information from the Sloan Digital Sky Survey, which is mapping galaxies with a telescope in New Mexico. The other team used figures from a sky survey called 2dF, which stands for the 2-degree field of view being surveyed by a telescope in Australia.

As the astronomers look farther away and further back in time, the size of the ripples will decrease in size. The ripples could serve as a convenient yardstick to track the history of the universe's expansion. That could shed light on dark energy, a mysterious cosmological force discovered in the past few years that, at cosmological distances, is stronger than gravity and is causing the expansion of the universe to accelerate.